Method of making killed resulphurized steel



United States PatentO 2,780,540 METHOD or Manner; KgLED RESULPHURIZED Earl W. Pierce, Chicago, Ill., assignor to United States Steel Corporation, a corporation of New Jersey No Drawing. Application April 9, 1954, Serial No. 422,235

3 Claims. (Cl. 75-129) This invention relates to steel production and, in particular, to process for making killed carbon or alloy steels in which sulphur is added after tapping from the melting furnace (open-hearth or electric-arc), in order to impart desired properties to the product.

In conventional steel-making methods, such as the open-hearth process, the content of sulphur is reduced progressively along with the carbon content. For certain uses, e. g., free-machining stock, it is desirable that the steel have a higher sulphur content than normally exists when the carbon has been brought down to the customary value. This has led to the practice of resulphurizing which involves producing a slag practically incapable of absorbing additional sulphur and, after tapping, adding sulficient sulphur to the steel in the ladle to raise the sulphur content to the final desired value, usually from .035 to as much as .150%.

Resulphurized steel possesses the free-machining quality and the mechanical properties desired for certain applications but is characterized by diificulty in hot-rolling. This characteristic causes cracks to form in billets rolled from ingots thereof, requiring an excessive amount of conditioning work on the billets before reduction to finished sizes. Even with the most careful conditioning, however, some seams escape detection and remain in the product. Such seams in the finished steel may open up when it is fabricated by the user. It is desirable, in many instances, to resulphurize killed steels but the added sulphur, even when comparatively small in amount, impairs the hot rolling characteristics of the metal.

I have discovered a method of making killed resulphurized steel which overcomes its tendency to crack during initial rolling, referred to above. Generally stated, my method consists in melting and working a heat until the sulphur content is reduced to the practical minimum, say under 030% and, after tapping from the furnace, finally killing and resulphurizing it, and then adding to it a small amount of an iron-silicon-magnesium alloy. This alloy is composed largely of iron and silicon in approximately equal amounts, and a minor amount, say one-tenth or one-twelfth, of magnesium. The proportions of the several elements of the alloy may vary considerably, i. e., the magnesium from 6 to 12% and the silicon and iron from 30 to 50% each. A satisfactory alloy composition is about 9% magnesium, about 45% silicon and the balance substantially iron, except for impurities usually present in such alloys, and occasionally small percentages of other elements, e. g., about cerium and about 1% aluminum.

The amount of the iron-silicon-magnesiurn alloy added my Patent No. 2,585,404 may conveniently be used.

For a clearer understanding of the invention, the following log is included showing the processing of a typical heat of low-alloy steel by my improved practice in a l20-ton open-hearth furnace:

'llme Operation Percent carbon l v Charged 21,100#lirncst0ne w 120 {Charged 147,300# scrap i 3:15 Charged 120,000# pig iron. 5:35 Melted {Added 660?? molybdenum oxldeunl Added 120011 raw lime 7:00 Added 1200;; raw lime. .72 Added 1000# lump ore .60 Added 900%; burnt lime" .31 Added 9-001; 50% lorrosilico 22 Added 2.500% ferromangancse Tapped By 8:30 the sulphur content had been reduced to 020%.

After tapping, the following ladle additions were made in the order given: 650$): silicon (as silicon alloy), 90:; sulphur, coal, 200# ferromanganese and 240?: aluminum shot. The steel was thus fully killed and resulphurized. Finally there was added 260# of magnesium ferrosilicon (9% Mg, 46% Si, balance substantially iron) and the steel was then teemed from the ladle into ingot molds whereupon it gave the following analysis: 28% carbon, .81% manganese, .Ol7% phosphorus, 043% sulphur, .31% silicon, 25% molybdenum and the balance substantially iron, except for the conventional amounts of residual elements.

While I have herein disclosed my improved method particularly in connection with the practice which is con ventional in using open-hearth furnaces, it may also be employed in the manufacture of open-hearth grades of steel in the electric furnace.

The steel produced by my invention exhibits greatly improved hot-working qualities compared to killed resulphurized steel as made heretofore. Experience with a substantial number of heats indicates that the steel produced by my invention encounters rejection for surface defects only to the extent of 1 to 4% whereas the comparable figure for conventionally killed resulphurized steel is about 20%. A similar improvement was noted in the fabrication of the product by the customer. In one case, a hot-forging operation, rejections because of steel defects were reduced from around 20% to between 1 and 2%.

Although I have disclosed herein the preferred practice of my invention, I intend to cover as well any change or modification therein which may be made Without departing from the spirit and scope of the invention.

I claim:

1. In the manufacture of steel, the method which consists in melting a heat of steel, working the heat to reduce the sulphur content to a practical minimum, tapping the heat, deoxidizing the steel, resulphurizing it, and then 3 adding to the resulphurized steel from 1 to 4 lb. per ton of an alloy composed of about one-tenth magnesium and substantially the remainder iron and silicon in approximately equal amounts, thereby giving the steel improved hot-working properties.

2. In the manufacture of steel, the method which con sists in deoxidizing and resulphurizing the steel after working a heat thereof down to a sulphur content lower than that desired in the finished product, then adding to the steel from 1 to 4 lb. per ton of an alloy composed of from 30 to 50% each of iron and silicon and from 6 to 12% ot'rnagnesium.

3. In the manufacture of steel, the step of adding to the finished steel While still molten after killing and resulphurizing, from 1 to 4 lb. per ton of an alloy composed of approximately equal amounts of iron and silicon and a minor amount of magnesium.

References Cited in the file of this patent UNITED STATES PATENTS Graham May 2, 1934 M55995 Austin July 7, 1936 

1. IN THE MANUFACTURE OF STEEL, THE METHOD WHICH CONSISTS IN MELTING A HEAT OF STEEL, WORKING THE HEAT TO REDUCE THE SULPHUR CONTENT TO A PRACTICAL MINIMUM, TAPPING THE HEAT, DEOXIDIZING THE STEEL, RESULPHURIZING IT, AND THEN ADDING TO THE RESULPHURIZED STEEL FROM 1 TO 4 LB. PER TON OF AN ALLOY COMPOSED OF ABOUT ONE-TENTH MAGNESIUM AND SUBSTANTIALLY THE REMAINDER IRON AND SILICON IN APPROXIMATELY EQUAL AMOUNTS, THEREBY GIVING THE STEEL IMPROVED HOT-WORKING PROPERTIES. 